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Podocytes are specialized, highly differentiated epithelial cells in the kidney glomerulus that are exposed to glomerular capillary pressure and possible increases in mechanical load. The proteins sensing mechanical forces in podocytes are unconfirmed, but the classic transient receptor potential channel 6 (TRPC6) interacting with the MEC-2 homolog podocin may form a mechanosensitive ion channel complex in podocytes. Here, we observed that podocytes respond to mechanical stimulation with increased intracellular calcium concentrations and increased inward cation currents. However, TRPC6-deficient podocytes responded in a manner similar to that of control podocytes, and mechanically induced currents were unaffected by genetic inactivation of TRPC1/3/6 or administration of the broad-range TRPC blocker SKF-96365. Instead, mechanically induced currents were significantly decreased by the specific P2X purinoceptor 4 (P2X4) blocker 5-BDBD. Moreover, mechanical P2X4 channel activation depended on cholesterol and podocin and was inhibited by stabilization of the actin cytoskeleton. Because P2X4 channels are not intrinsically mechanosensitive, we investigated whether podocytes release ATP upon mechanical stimulation using a fluorometric approach. Indeed, mechanically induced ATP release from podocytes was observed. Furthermore, 5-BDBD attenuated mechanically induced reorganization of the actin cytoskeleton. Altogether, our findings reveal a TRPC channel-independent role of P2X4 channels as mechanotransducers in podocytes., (Copyright © 2016 by the American Society of Nephrology.)

Specific biological roles of the classical transient receptor potential channel 1 (TRPC1) are still largely elusive. To investigate the function of TRPC1 proteins in cell physiology, we studied heterologously expressed TRPC1 channels and found that recombinant TRPC1 subunits do not form functional homomeric channels. Instead, by electrophysiological analysis TRPC1 was shown to form functional heteromeric, receptor-operated channel complexes with TRPC3, -4, -5, -6, and -7 indicating that TRPC1 proteins can co-assemble with all members of the TRPC subfamily. In all TRPC1-containing heteromers, TRPC1 subunits significantly decreased calcium permeation. The exchange of select amino acids in the putative pore-forming region of TRPC1 further reduced calcium permeability, suggesting that TRPC1 subunits contribute to the channel pore. In immortalized immature gonadotropin-releasing hormone neurons endogenously expressing TRPC1, -2, -5, and -6, down-regulation of TRPC1 resulted in increased calcium permeability and elevated basal cytosolic calcium concentrations. We did not observe any involvement of TRPC1 in store-operated cation influx. Notably, TRPC1 suppressed the migration of gonadotropin-releasing hormone neurons without affecting cell proliferation. Conversely, in TRPC1 knockdown neurons, specific migratory properties like distance covered, locomotion speed, and directionality were increased. These findings suggest a novel regulatory mechanism relying on the expression of TRPC1 and the subsequent formation of heteromeric TRPC channel complexes with reduced calcium permeability, thereby fine-tuning neuronal migration.

The mammalian TRPM gene family can be subdivided into distinct categories of cation channels that are either highly permeable for Ca(2+) (TRPM3/6/7), nonselective (TRPM2/8), or even Ca(2+) impermeable (TRPM4/5). TRPM6/7 are fused to alpha-kinase domains, whereas TRPM2 is linked to an ADP-ribose phosphohydrolase (Nudix domain). At a molecular level, the evolutionary steps that gave rise to the structural and functional TRPM channel diversity remain elusive. Here, we provide phylogenetic evidence that Nudix-linked channels represent an ancestral type of TRPMs that is present in various phyla, ranging from protists to humans. Surprisingly, the pore-forming segments of invertebrate TRPM2-like proteins display high sequence similarity to those of Ca(2+)-selective TRPMs, while human TRPM2 is characterized by a loss of several conserved residues. Using the patch-clamp technique, Ca(2+) imaging, and site-directed mutagenesis, we demonstrate that restoration of only two "ancient" pore residues in human TRPM2 (Q981E/P983Y) significantly increased (approximately 4-fold) its permeability for Ca(2+). Conversely, introduction of a "modern" sequence motif into mouse TRPM7 (E1047Q/Y1049P) resulted in the loss of Ca(2+) permeation and a linear TRPM2-like current-voltage relationship. Overall, our findings provide an integrative view on the evolution of the domain architecture and the structural basis of the distinct ion permeation profiles of TRPM channels.

The mammalian transient receptor potential (TRP) superfamily of cation channels can be divided into six major families. Among them, the "classical" or "canonical" TRPC family is most closely related to Drosophila TRP, the founding member of the superfamily. All seven channels of this family designated TRPC1-7 share the common property of activation through phospholipase C (PLC)-coupled receptors, but their gating by receptor- or store-operated mechanisms is still controversial. The TRPC3, 6, and 7 channels are 75% identical and are also gated by direct exposure to diacylglycerols (DAG). TRPC3, 6, and 7 interact physically and, upon coexpression, coassemble to form functional tetrameric channels. This review will focus on the TRPC3/6/7 subfamily and describe their functional properties and regulation as homomers obtained from overexpression studies in cell lines. It will also summarize their heteromultimerization potential in vitro and in vivo and will present preliminary data concerning their physiological functions analyzed in isolated tissues with downregulated channel activity and gene-deficient mouse models.

The classical transient receptor potential (TRP) protein family consists of seven members which share a common gating mechanism contingent on phospholipase C activation. While some family members are thought to be activated subsequent to emptying of intracellular calcium stores, others appear to be gated by as yet undefined lipid messengers. TRPC 3, 6 and 7 form a structural and functional TRPC subfamily characterized by their sensitivity towards diacylglycerols (DAGs). TRPC6 is a non-selective cation channel that is activated by DAG in a membrane-delimited fashion, independently of protein kinase C. Depletion of internal Ca2+ stores is not required for TRPC6 activity. TRPC6 mRNA and protein are abundantly expressed in smooth muscle cells and DAG-evoked Ca2+ transients can be observed in primary myocytes derived from lung and blood vessels. Thus, TRPC6 is a promising candidate for as yet unidentified non-selective cationic channels in smooth muscle cells potentially involved in vasoconstrictor-activated cation influx and myogenic tone of resistance arteries. Recent systematic studies revealed that TRPC proteins assemble into heteromultimers predominantly within the confines of distinct TRPC subfamilies. The known principles of channel complex formation will be instrumental in assessing the physiological role of distinct TRPC proteins in living cells.

To study the role of endothelial ATP-sensitive K+ channels in the regulation of vascular tone we examined the intracellular calcium concentration ([Ca2+]i) in coronary capillaries consisting only of endothelial cells. Coronary capillary fragments were isolated enzymatically from the guinea-pig heart and [Ca2+]i was determined by microfluorometry of fura-2 loaded cells. Low concentrations of the K+ channel opener diazoxide, which caused pronounced glibenclamide-sensitive hyperpolarization in capillaries, induced a rapid, transient rise in [Ca2+]i followed by a sustained elevation of [Ca2+]i (19 of 40 experiments). [Ca2+]i in the endothelial cells increased from 32 +/- 7 nM at rest to 66 +/- 11 nM at the peak (n = 19). One third of the [Ca2+]i-transients showed irregular oscillations of [Ca2+]i. No significant difference in the [Ca2+]i-response induced by 100 nM or 1 muM diazoxide was found. Similar results were obtained with the K+ channel opener rilmakalim. Simultaneous measurements of the membrane potential and [Ca2+]i with fluorometric methods indicated that the hyperpolarization but not the [Ca2+]i-transient could be repeatedly induced in a single capillary by the K+ channel openers. Electrophysiological recordings of the membrane potential using the "perforated patch" method (n = 4), showed that rilmakalim (1 muM) induced hyperpolarization of capillaries towards the K+ equilibrium potential, confirming our fluorometric measurements. In conclusion, for the first time, these data indicate that K+ channel openers induce [Ca2+]i-transients in microvascular endothelial cells. This raises the possibility that these drugs not only act as synthetic vasoactive factors via hyperpolarizing smooth muscle cells but also via NO release of microvascular endothelial cells. Interestingly, only 100 nM diazoxide was sufficient for a maximal response, suggesting the expression of a new type of KATP-channel in coronary capillaries characterised by high sensitivity to diazoxide.

To study the role of endothelial ATP-sensitive K+ channels in the regulation of vascular tone we examined the intracellular calcium concentration ([Ca2+]i) in coronary capillaries consisting only of endothelial cells. Coronary capillary fragments were isolated enzymatically from the guinea-pig heart and [Ca2+]i was determined by microfluorometry of fura-2 loaded cells. Low concentrations of the K+ channel opener diazoxide, which caused pronounced glibenclamide-sensitive hyperpolarization in capillaries, induced a rapid, transient rise in [Ca2+]i followed by a sustained elevation of [Ca2+]i (19 of 40 experiments). [Ca2+]i in the endothelial cells increased from 32 ± 7 nM at rest to 66 ± 11 nM at the peak ( n = 19). One third of the [Ca2+]i-transients showed irregular oscillations of [Ca2+]i. No significant difference in the [Ca2+]i-response induced by 100 nM or 1 μM diazoxide was found. Similar results were obtained with the K+ channel opener rilmakalim. Simultaneous measurements of the membrane potential and [Ca2+]i with fluorometric methods indicated that the hyperpolarization but not the [Ca2+]i-transient could be repeatedly induced in a single capillary by the K+ channel openers. Electrophysiological recordings of the membrane potential using the ”perforated patch” method ( n = 4), showed that rilmakalim (1 μM) induced hyperpolarization of capillaries towards the K+ equilibrium potential, confirming our fluorometric measurements. In conclusion, for the first time, these data indicate that K+ channel openers induce [Ca2+]i-transients in microvascular endothelial cells. This raises the possibility that these drugs not only act as synthetic vasoactive factors via hyperpolarizing smooth muscle cells but also via NO release of microvascular endothelial cells. Interestingly, only 100 nM diazoxide was sufficient for a maximal response, suggesting the expression of a new type of KATP-channel in coronary capillaries characterised by high sensitivity to diazoxide. [ABSTRACT FROM AUTHOR]